Currently 2-D Dual-energy X-ray Absorptiometry is the gold standard of osteoporosis assessment. Dual-energy X-ray absorptiometry (DXA) is a means of performing skeletal assessment using x-ray beams of differing energy levels. A DXA system generally includes an examination table and a C-arm at opposite ends of which are mounted an x-ray tube and a multi-detector array. The patient is positioned on the examination table between the x-ray tube and the detector array of the C-arm and a fan-shaped beam of x-rays are directed from the tube to the detector as the tube and detector are scanned axially along the patient. The X-rays are alternatively pulsed at high and low energy ranges. By comparing the relative attenuation of the x-rays at the two energies, the contributions to the attenuation due to the soft tissue can be subtracted. When soft tissue absorption is subtracted out, a two dimensional image of the bony structure of the patient remains and may be used for skeletal assessment purposes.
DXA has grown to be the gold standard for osteoporosis assessment because it provides improved skeletal assessment results (in comparison to single energy x-rays) while minimizing patient exposure to radiation. However one problem with DXA is that it fails to provide important 3-D information about the volumetric density and spatial geometry of imaged skeletal structures. Such 3-D information may be obtained using Quantitative Computed Tomography (QCT) Densitometry. The QCT examination is performed using a CT scanner to obtain volumetric data. One issue with QCT lies in whether the quality of the information obtained by 3D structural evaluation using QCT outweighs the patient's increased exposure to radiation, the financial costs associated with QCT and the clinical feasibility of actual QCT practice.
Attempts have been made to leverage the information provided by DXA to obtain skeletal assessment information of similar quality to that provided by QCT. These attempts have involved selecting a standard 3-D model of a skeletal structure of a patient from an atlas of structures, and customizing the 3-D model by varying its modes of deformation until digitally reconstructed radiographs (DDRs) (i.e., 2-D projection images) of the 3-D model can be ‘matched’ to co-registered DXA projection images of the patient. With such an arrangement a customized 3-D VXA model of a skeletal structure may be obtained using only a handful of DXA projection images, thereby minimizing patient exposure while theoretically providing improved quality skeletal assessment data.
However it has been determined that not all of the skeletal health assessment information provided by a customized VXA model meets the quality standards of QCT. Customized VXA models have shown great accuracy for skeletal assessment measurements related to the spatial geometry of the structure. However, measurements which attempt to quantify the volumetric density of the skeletal structure, and in particular the Bone Mineral Density (BMD), have proven less reliable. It would be desirable to identify a method for obtaining volumetric density information having accuracy comparable to that provided by QCT yet at DXA dosing levels.